I went thru the same problem trying to figure mine out. Laramie, Alpine 9 w/ 8.4 non nav. As far I can tell base share the same speakers, 2 ohm, 6x9 blue poly cone 050643584ab. Use the lowest possible you can get/afford. You are going to have to change the dash or add an amp/rewire for to handle the 8 ohm, they over will tax the head unit amp. I think base 6 frt door & dash are run off the same feed. I didn't close look at or save the wiring diag for base 6.
I went with infinity ref 3032cfx 3 ohm for dash(3). Infinity kappa 93ix 2.5 ohm (base response down to 35hz, most 6x9s are 40 or 45+) for all 4 doors, some light mods needed to mount, & alpine front doors only get low pass from amp. My goal was really good sound & extra bass add (8" stk sub little weak), not have to add or bypass anything, budget friendly. All achieved!!! Kicks *** above 30 now, even above 20, leaves ears ringing after a bit. But that said alpine has at least a small amp which can make a big diff. I'm going to try the extra dash in the headliner to see what it does, probably order another set to get one more.